MONDian dynamical modelling of NGC 288 with β ≠ 0

Abstract

NGC 288 is a diffuse Galactic globular cluster, it is remarkable in that its low density results in internal accelerations being below the critical MOND a 0 acceleration throughout. This makes it an ideal testing ground for MONDian gravity, as the details of the largely unknown transition function between the Newtonian and modified regimes become unimportant. Further, exact analytical solutions exist for isothermal spherical equilibrium structures in MOND, allowing for arbitrary values of the anisotropy parameter, β. In this paper we use observations of the velocity dispersion profile of NGC 288, which is in fact isothermal, as dynamical constraints on MONDian models for this cluster, where the remaining free parameters are adjusted to fit the observed surface brightness profile. We find the optimal fit requires β = 0, an isotropic solution with a total mass of 3.5 ± 1.1 × 10 4 M ⊙ .

MIKiS: The Multi-instrument Kinematic Survey of Galactic Globular Clusters. I. Velocity Dispersion Profiles and Rotation Signals of 11 Globular Clusters*

“…Particularly interesting is the case of NGC 288, for which we detect a clear decline beyond r ∼ 200 ′′ (corresponding roughly to 9 pc for the distance quoted in Ferraro et al 1999; see Table 10). This is not in agreement with the results of Hernandez et al (2017), who find that the VD profile of this cluster flattens at r ∼ 8-10 pc and stays constant at σ P ≃ 2.0 km/s over the whole radial range sampled. It worth noticing, however, that the VD curve of Hernandez et al (2017) is obtained from 148 stars with cluster-centric distances r ≤ 16 pc, while our results is based on a sample of more than 400 members observed out to ∼ 30 pc.…”

“…This is not in agreement with the results of Hernandez et al (2017), who find that the VD profile of this cluster flattens at r ∼ 8-10 pc and stays constant at σ P ≃ 2.0 km/s over the whole radial range sampled. It worth noticing, however, that the VD curve of Hernandez et al (2017) is obtained from 148 stars with cluster-centric distances r ≤ 16 pc, while our results is based on a sample of more than 400 members observed out to ∼ 30 pc. On the other hand, the observed declining shape of the VD profile is agreement with the results of Lane et al (2010) and Kimmig et al (2015), and it is well matched by the King model that best fits the star density distribution of the cluster.…”

Testing modified-gravity theories via wide binaries and GAIA

“…Following on from earlier related studies (e.g. Hernandez et al (2011), Hernandez et al (2012, Hernandez et al (2017)), we have estimated the prospects for observational tests of modified-gravity theories using wide-binary stars selected by GAIA and high-precision radial velocities from groundbased telescopes. Considering the ratio of 3-D relative velocities to the Newtonian circular velocity, in standard gravity the probability distribution function contains a rather steep decline at v3D/vC (r) ∼ 1.2, resulting in 80th and 90th percentile values which are only weakly dependent on the uncertain distribution of orbital eccentricities.…”

Asymptotic kinematics of globular clusters: The emergence of a Tully–Fisher relation

“…This has been interpreted as evidence in favour of a modified gravity regime appearing in the a < a0 regime by e.g. Hernandez et al (2013) and Hernandez et al (2017), but explained under a Newtonian scenario through selecting initial conditions in terms of density profiles, binary fractions and distributions and initial stellar mass functions for dynamical models, which evolve into the situation observed today. Examples of the latter case include Claydon et al (2017) who consider the contribution of unbound stars to the resulting present day velocity dispersion profiles, obtaining an outer flattening consistent with observations, as was also found by Kennedy (2014) by considering chaotic internal dynamics resulting from the interaction with the overall galactic potential, an effect which naturally increases with decreasing GC mass.…”